3.4.5.1. Reference Conditions

Anthropogenic emissions of chlorofluorocarbons (freons) and some other substances
into the atmosphere are known to deplete the stratospheric ozone layer (Albritton
and Kuijpers, 1999). This layer absorbs ultraviolet solar radiation within a
wavelength range of 280-320 nm (UV-B), and its depletion leads to an increase
in ground-level flux of UV-B radiation (Herman et al., 1996; Jackman
et al., 1996; McPeters et al., 1996; Madronich et al.,
1998; McKenzie et al., 1999). Enhanced UV-B suppresses the immune system
and may cause skin cancer in humans and eye damage in humans and other animal
species (Diffey, 1992; de Gruijl, 1997; Longstreth et al., 1998). It
can affect terrestrial and marine ecosystems (IASC, 1995; Zerefos and Alkiviadis,
1997; Caldwell et al., 1998; Hader et al., 1998; Krupa et al.,
1998) and biogeochemical cycles (Zepp et al., 1998) and may reduce the
service life of natural and synthetic polymer materials (Andrady et al.,
1998). It also interacts with other atmospheric constituents, including GHGs,
influencing radiative forcing of the climate (see TAR WGIChapters 4, 6, and 7).

Analyses of ozone data and depletion processes since the early 1970s have shown
that the total ozone column has declined in northern hemisphere mid-latitudes
by about 6% in winter/spring and 3% in summer/autumn, and in southern hemisphere
mid-latitudes by about 5% on a year-round basis. Spring depletion has been greatest
in the polar regions: about 50% in the Antarctic and 15% in the Arctic (Albritton
and Kuijpers, 1999). These five values are estimated to have been accompanied
by increases in surface UV-B radiation of 7, 4, 6, 130, and 22%, respectively,
assuming other influences such as clouds to be constant. Following a linear
increase during the 1980s, the 1990s springtime ozone depletion in Antarctica
has continued at about the same level each year. In contrast, a series of cold,
protracted winters in the Arctic have promoted large depletions of ozone levels
during the 1990s (Albritton and Kuijpers, 1999).

3.4.5.2. Development and Application of UV-B Scenarios

Scenarios of the future thickness of the ozone column under given emissions
of ozone-depleting gases can be determined with atmospheric chemistry models
(Alexandrov et al., 1992; Brasseur et al., 1998), sometimes in
combination with expert judgment. Processes that affect surface UV-B flux also
have been investigated via models (Alexandrov et al., 1992; Matthijsen
et al., 1998). Furthermore, several simulations have been conducted with
coupled atmospheric chemistry and climate models, to investigate the relationship
between GHG-induced climate change and ozone depletion for different scenarios
of halogenated compounds (Austin et al., 1992; Shindell et al.,
1998). It is known that potential stratospheric cooling resulting from climate
change may increase the likelihood of formation of polar stratospheric clouds,
which enhance the catalytic destruction of ozone. Conversely, ozone depletion
itself contributes to cooling of the upper troposphere and lower stratosphere
(see TAR WGIChapter 7).

Serious international efforts aimed at arresting anthropogenic emissions of
ozone-depleting gases already have been undertakennamely, the Vienna Convention
for the Protection of the Ozone Layer (1985) and the Montreal Protocol on Substances
that Deplete the Ozone Layer (1990) and its Amendments. The abundance of ozone-depleting
gases in the atmosphere peaked in the late 1990s and now is expected to decline
as a result of these measures (Montzka et al., 1996), recovering to pre-1980
levels around 2050 (Albritton and Kuijpers, 1999). Without these measures, ozone
depletion by 2050 was projected to exceed 50% in northern mid-latitudes and
70% in southern mid-latitudesabout 10 times larger than today. UV-B radiation
was projected to double and quadruple in northern and southern mid-latitudes,
respectively (Albritton and Kuijpers, 1999).

There have been numerous experimental artificial exposure studies of the effects
of UV-B radiation on plants (Runeckles and Krupa, 1994). There also have been
a few investigations of the joint effects of enhanced UV-B and other environmental
changes, including climate (Unsworth and Hogsett, 1996; Gwynne-Jones et al.,
1997; Sullivan, 1997). A study of the impacts of UV-B on skin cancer incidence
in The Netherlands and Australia to 2050, using integrated models, is reported
by Martens (1998), who employed scenarios of future ozone depletion based on
the IS92a emissions scenario and two scenarios assuming compliance with the
London and Copenhagen Amendments to the Montreal Protocol.